1. Field of the Invention
The present invention relates to a coherent laser radar system and a target measurement method for measuring physical information such as a target distance, velocity, density distribution and velocity distribution of the target, and particularly to a coherent laser radar system and a target measurement method utilizing a pulsed laser oscillating a single-wavelength (single-frequency) pulsed laser beam as a light source.
2. Description of Related Art
As devices for measuring physical information such as the target distance, velocity, density distribution and velocity distribution of the target, there are a pulse Doppler radar system utilizing microwaves or millimeter waves and a coherent laser radar system utilizing light waves (laser beam). Because of the difference between their frequencies, the former can perform wide-range, long-distance measurements, whereas the latter can perform measurements at high spatial resolution and high velocity resolution.
In soft target measurements, such as measurements of wind velocity and wind velocity distribution, the pulse Doppler radar system handles raindrops and particles of mist or cloud in atmosphere as scatterers, and computes the wind velocity from the Doppler shift of the echo. Accordingly, it is difficult for the pulse Doppler radar system to measure the clear-air turbulence because not enough echo is captured in clear weather in which there are no raindrops, particles of mist or cloud in the atmosphere.
In contrast with this, the coherent laser radar system can measure the wind velocity and wind velocity distribution even in clear weather because it utilizes the laser beam and hence can achieve enough scattering intensity in aerosol in the atmosphere. Thus, the coherent laser radar system installed in an airport or aircraft is expected to serve as a device for detecting obstacles such as turbulence. There are two types of coherent laser radar systems: one employs as its light source a pulsed laser that oscillates a single frequency pulsed laser beam; and the other uses as its light source a continuous wave (CW) laser that oscillates a single frequency continuous laser beam.
FIG. 16 is a block diagram showing a configuration of a conventional coherent-laser radar system disclosed in U.S. Pat. No. 5,237,331, for example. The conventional coherent laser radar system utilizes an injection-seeding pulsed laser as its light source.
In FIG. 16, the reference numeral 101 designates a CW laser light source for oscillating a single-frequency CW laser beam; 102 designates an optical divider for dividing part of the CW laser beam as a local beam; 103 designates a frequency shifter for shifting the frequency of the CW laser beam; 104 designates an injection-seeding pulsed laser for generating a pulsed laser beam utilizing the CW laser beam as a seed beam; 105 designates an optical divider for dividing the pulsed laser beam; 106 designates a beam splitter for reflecting the light beam supplied from the optical divider 105 using the difference in polarization direction, and for transmitting the light beam supplied from a quarter-wave plate 107; 107 designates the quarter-wave plate for converting a linearly polarized beam with a certain polarization direction with respect to the crystallographic axis to a circularly polarized beam, and for converting a circularly polarized beam into a linearly polarized beam; 108 designates a transceiver optics for supplying a scanning optics 109 with a beam from the quarter-wave plate 107, and for supplying the quarter-wave plate 107 with a beam from the scanning optics 109 along the same optical path; and 109 designates the scanning optics for transmitting a transmitted beam to a target, and for receiving a scattered beam from the target as a received beam.
The reference numeral 110 designates an optical divider for dividing the local beam; 111 designates an optical coupler for coupling the local beam divided by the optical divider 110 with the pulsed laser beam divided by the optical divider 105; 112 designates an optical coupler for coupling the local beam divided by the optical divider 110 with the received beam passing through the beam splitter 106; 113 designates a photodetector for detecting a light beam output from the optical coupler 111; 114 designates a photodetector for detecting a light beam output from the optical coupler 112; 115 designates an A/D converter for converting the electric signals which are detected and generated by the photodetectors 113 and 114 into digital signals; and 116 designates a signal processor for computing the physical information such as the target distance, velocity, density distribution and velocity distribution in response to the two digital detection signals output from the A/D converter 115.
The reference numeral 117 designates a controller for controlling an adjuster 118 in response to the signal supplied from the signal processor 116; and 118 designates the adjuster such as a piezoelectric device for adjusting the cavity length of the injection-seeding pulsed laser 104.
FIG. 17 is a block diagram showing a configuration of the signal processor 116 of the conventional coherent laser radar system. In this figure, the reference numeral 121 designates a memory unit for temporarily storing the digital signals fed from the A/D converter 115; 122 designates a time gate for selecting from the digital signals stored in the memory unit 121 the digital signals corresponding to the received beam from a particular range; 123 designates a window processor for executing window processing such as Hanning window processing or Hamming window processing; 124 designates an FFT section for carrying out fast Fourier transform (FFT); and 125 designates a Doppler frequency detector for detecting the Doppler frequency in response to the signal passing through the Fourier transform.
Next, the operation of the conventional radar system will be described.
FIG. 18 is a timing chart illustrating the operation of the conventional coherent laser radar system.
The CW laser light source 101 oscillates the CW laser beam at a single frequency f0 (that is, at a single wavelength), and supplies it to the optical divider 102. The optical divider 102 divides the CW laser beam into two portions. A first one of the two CW laser beams is used as the local beam, and the second one is supplied to the frequency shifter 103. The local beam is further divided into two portions by the optical divider 110, and they are supplied to the optical couplers 111 and 112. On the other hand, the frequency shifter 103 increases the frequency of the CW laser beam by fIF, and supplies the injection-seeding pulsed laser 104 with the CW laser beam with the frequency f0+fIF as the seed beam.
The injection-seeding pulsed laser 104 oscillates the single frequency (that is, single wavelength) pulsed laser beam in the axial mode at a frequency closest to the seed beam. The pulsed laser beam output from the injection-seeding pulsed laser 104 is divided by the optical divider 105 into two parts, and a first part is incident on the beam splitter 106, whereas a second part is incident on the optical coupler 111.
The pulsed laser beam output from the injection-seeding pulsed laser 104, linearly polarized beam with a particular polarization direction, is reflected off the beam splitter 106 and is incident on the quarter-wave plate 107. The quarter-wave plate 107 converts it to the circularly polarized beam which is transmitted the to a target as the transmitted beam via the transceiver optics 108 and the scanning optics 109.
The pulsed laser beam thus transmitted to the target is scattered by the target, and part of the scattered beams is incident on the scanning optics 109.
The scattered beam from the target, that is, the received beam, reversely proceeds along the same optical path as the transmitted beam through the scanning optics 109 and the transceiver optics 108, and is incident on the quarter-wave plate 107. The quarter-wave plate 107 rotates the polarization direction of the received beam so that it becomes a linearly polarized beam whose polarization direction is rotated by 90 degrees with respect to the pulsed laser beam, and supplies it to the beam splitter 106.
The beam splitter 106 transmits the received beam, and supplies it to the optical coupler 112. The optical coupler 112 couples the received beam with the local beam, and supplies the coupled beam to the photodetector 114. The photodetector 114 carries out the coherent detection of the coupled beam, and supplies the A/D converter 115 with the electric signal generated by the detector.
On the other hand, the optical coupler 111 couples the pulsed laser beam split by the optical divider 105 with the local beam, and supplies the coupled beam to the photodetector 113. The photodetector 113 carries out the coherent detection of the coupled beam, and supplies the electric signal generated by the detector to the A/D converter 115.
The A/D converter 115 samples the electric signals fed from the photodetectors 113 and 114, and supplies them to the signal processor 116 as the digital signals. The signal processor 116 computes in response to the signal from the photodetector 114 the target distance from the temporal waveform of the signal intensity and the target velocity from the Doppler signal component of the signal.
To achieve accurate measurement, the frequency of the local beam and that of the pulsed laser beam must have a fixed relationship during the sampling. Thus, the CW laser light source 101 must have a high frequency stability. For example, assuming that the CW laser beam used as the local beam has a wavelength of 2 micrometers, the maximum measurement range is 15 kilometers, and the measurement error of the target velocity is less than 0.1 m/s, the frequency fluctuations of the CW laser beam in 0.1 millisecond must be less than 100 kHz. To achieve such a high frequency stability, it is necessary to use a CW laser with a complicated structure, to select an appropriate wavelength and to maintain the temperature stability and power supply stability at a high level.
The signal processor 116 stores into the memory unit 121 the signal which corresponds to the coupled beam of the received beam and local beam sampled by the A/D converter 115 (the beat signal shown in FIG. 18). The sampling duration of the A/D converter 115 is from the pulse oscillation to the reception of the scattered beam from a target located at the maximum measurement range.
To obtain the information about the target at a given distance, the time gate 122 extracts from the sampled signal stored in the memory unit 121 a portion including the scattered beam from the target, and supplies it to the window processor 123. The window processor 123 performs the window processing of the sampled signal to improve the frequency measurement accuracy. Thus, the signals with waveforms as shown in FIG. 18 are obtained.
The FFT section 124 calculates the spectrum of the signal after the window processing. The Doppler frequency detector 125 detects the Doppler frequency and computes the target velocity.
In this way, the conventional coherent laser radar system measures the target velocity and the like.
The signal output from the photodetector 113 is used to improve the calculation accuracy of the target distance and velocity. As described above, since the injection-seeding pulsed laser 4 oscillates the pulse at the frequency closest to the frequency of the seed beam in the axial mode, it is necessary to monitor the frequency difference between the pulsed laser beam and the local beam to obtain an accurate Doppler signal. To achieve this, the optical dividers 105 and 110 extract part of the pulsed laser beam and part of the local beam, respectively, and the photodetector 113 carries out the coherent detection of the signal generated by coupling the two signals by the optical coupler 111. Furthermore, the A/D converter 115 samples the detected signal in the same manner as the received beam, and the signal processor 116 computes the frequency difference between the pulsed laser beam and the local beam, and supplies the controller 117 with the signal for controlling the frequency of the pulsed laser beam in response to the calculation result.
The frequencies of the seed beam, pulsed laser beam, received beam, frequency monitoring signal and received signal fS, fT, fR, fM and fsig can be expressed as follows.
fS=fo+fIF
fT=fS+xcex94f
fR=fT+fd
fM=fIF+xcex94f
fsig=fM+fd
where f0 is the frequency of the local beam, xcex94f is the frequency difference between the pulsed laser beam and the seed beam and fdis the target Doppler frequency. The frequency monitoring signal is used for stabilizing the oscillation frequency of the pulsed laser beam.
Accordingly, the target Doppler frequency fdbecomes the difference between the frequency fsig of the received signal and the frequency fM of the frequency monitoring signal.
To achieve the stable injection seeding operation, the controller 117 controls the adjuster 118 that regulates the cavity length of the injection-seeding pulsed laser 104, thereby adjusting the frequency of the pulsed laser beam. The signal processor 116 supplies the controller 117 with the frequency difference xcex94f between the pulsed laser beam and the local beam in response to the frequency fM of the frequency monitoring signal. The controller 117 controls the adjuster 118 such that the frequency difference xcex94f becomes less than a predetermined value or zero, thereby regulating the cavity length of the injection-seeding pulsed laser 104 to adjust the frequency of the pulsed laser beam.
In this way, the injection-seeding pulsed laser 104 produces a stable single mode (single wavelength) pulsed laser beam.
With the foregoing configuration, it is necessary for the conventional coherent laser radar system to comprise the CW laser as the local light source besides the pulsed laser for generating the transmitted beam to perform the coherent detection. In addition, since the CW laser must oscillate the single frequency CW laser beam at a high frequency stability, it becomes complicated in its structure. Moreover, the mechanism is necessary for monitoring the frequency difference between the transmitted pulsed laser beam and the local beam. As a result, the configuration of the system becomes complex, which presents a problem of making it difficult to reduce the cost and size of the system and to increase its reliability.
The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to provide a simple structure, low cost, small size and highly reliable coherent laser radar system. This is implemented by transmitting a single frequency (single wavelength) pulsed laser beam to a target and receiving its response from the target, by delaying part of the pulsed laser beam and coupling it with the received beam, and by carrying out coherent detection of the coupled beam, thereby obviating the need for the CW laser light source and the mechanism for monitoring the frequency difference between the transmitted pulsed laser beam and the local beam.
According to a first aspect of the present invention, there is provided a coherent laser radar system comprising: a pulsed laser for oscillating a single wavelength pulsed laser beam; optical dividing means for dividing the pulsed laser beam oscillated by the pulsed laser, and for outputting first part of the pulsed laser beam as a transmitted beam and second part of the pulsed laser beam as a local beam; transceiver optical means for transmitting the transmitted beam to a target, and for receiving a light beam from the target as a received beam; a delay line for delaying the local beam output from the optical dividing means; optical coupling means for coupling the received beam with the local beam output from the delay line; a photodetector for carrying out coherent detection of a light beam output from the optical coupling means; and a signal processor for obtaining physical information about the target from the signal passing through the coherent detection by the photodetector.
Here the delay line may comprise an optical fiber with a predetermined length.
The delay line may comprise: an optical divider for dividing the local beam output from the optical dividing means into n light signals, where n is a positive integer greater than one; n delay lines for providing the n light signals with different delay times; and an optical coupler for coupling the n light signals delay by the n delay lines.
The delay line may comprise: a loop line; and an optical coupler for guiding the local beam output from the optical dividing means into the loop line, and for dividing part of the local beam traveling around the loop line.
The delay line may comprise an optical amplifier at a midpoint of the loop line.
The coherent laser radar system may further comprise an optical amplifier connected between the delay line and the optical coupling means for amplifying the local beam.
The optical amplifier may be an optical fiber amplifier.
The optical amplifier may be a semiconductor optical amplifier.
The optical amplifier may be a laser amplifier composed of a solid-state laser medium.
The optical coupler may be a variable optical coupler whose dividing ratio is variable.
The optical coupler may comprise: an optical switch consisting of an acoustooptic device for supplying the loop line with one of the local beam output from the optical dividing means and the local beam traveling around the loop line; and an optical divider for dividing the local beam traveling around the loop line.
The coherent laser radar system may further comprise an A/D converter for converting the signal output from the photodetector into digital data, and for supplying the digital data to the signal processor.
The signal processor may comprise: a time gate for enabling the digital data output from the A/D converter to be input to the signal processor as information data in response to the local beam; an FFT (fast Fourier transform) section for computing a spectrum of the information data; and a Doppler frequency detector for detecting a Doppler frequency and for computing the physical information of the target from the spectrum output from the FFT section.
According to a second aspect of the present invention, there is provided a target measurement method comprising the steps of: oscillating a single wavelength pulsed laser beam; dividing the pulsed laser beam into a first part used as a transmitted beam and a second part used as a local beam; transmitting the transmitted beam to a target, and receiving a light beam from the target as a received beam; delaying the local beam; coupling the received beam with the local beam delayed; carrying out coherent detection of the coupled light beam; and obtaining physical information about the target from the signal passing through the coherent detection.